1. Field of the Invention
The present invention relates in general to a mold assembly for forming a lens blank, and more particularly to a mold assembly for forming a lens blank for an ophthalmic lens such as a contact lens and an intraocular lens, such that the lens blank has desired configuration and dimensions.
2. Discussion of the Related Art
A known method of manufacturing an ophthalmic lens such as a contact lens includes a molding process for forming a lens blank using a mold assembly, and a cutting or machining process for cutting the lens blank into the desired ophthalmic lens. The mold assembly includes an upper mold and a lower mold which cooperate to define a fluid-tightly enclosed mold cavity. In the molding process, a liquid monomer composition filling the mold cavity is polymerized into the lens blank. In the cutting process, the thus formed lens blank is cut on its inner and outer surfaces, to produce the desired contact lens or other ophthalmic lens which has an inner or back surface for contact with the cornea of an eye, and an outer or front surface opposite to the inner surface.
JP-A-4-290706 (U.S. Pat. No. 5,137,441) discloses an example of a mold assembly used in a molding process as described above. This mold assembly consists of an upper mold, a lower mold, and a generally cylindrical intermediate sleeve which removably connect the inner and outer molds. The intermediate sleeve cooperates with the upper mold to define a liquid reservoir for storing an excess amount of the liquid monomer composition which has been discharged from a mold cavity upon closing of the mold assembly. The mold assembly is further adapted to permit a flow of the liquid monomer composition from the liquid reservoir into the mold cavity in the molding process, when a mass of the liquid monomer composition in the mold cavity shrinks during polymerization of the composition, so that a shortage in volume of the liquid monomer composition in the mold cavity is compensated for by a volume of the composition which has been supplied from the liquid reservoir into the mold cavity. Thus, the mold assembly is constructed to prevent a conventionally experienced problem which would otherwise take place due to shrinkage of the liquid monomer composition during polymerization thereof in the mold cavity.
The mold assembly disclosed in the above-identified publication is constructed to be assembled by moving the upper mold into the intermediate sleeve in the axial direction such that the outer circumferential surface of the upper mold is in sliding contact with the inner circumferential surface of the intermediate sleeve. However, the upper mold and the intermediate sleeve do not have an axial stopper for determining an axial position of the upper mold relative to the intermediate sleeve, namely, a relative position between the upper mold and the lower mold which is positioned in a lower portion of the intermediate sleeve. Accordingly, it is difficult to accurately establish the predetermined relative axial position of the upper and lower molds.
The relative axial position of the upper and lower molds determines an axial dimension of the mold cavity defined between the upper and lower molds in the mold assembly in the closed state. Therefore, it is difficult to maintain the axial dimension of the mold cavity at a predetermined constant value, during successive molding operations using the mold assembly, resulting in a high risk of variation in the thickness value of the lens blanks to be formed by the mold assembly. Further, the absence of the axial stopper in the mold assembly may cause a variation in the volume of the liquid reservoir defined by the upper mold and the intermediate sleeve, giving rise to an insufficient amount of supply of the liquid monomer composition from the liquid reservoir into the mold cavity due to an insufficient volume of the liquid reservoir, upon shrinking of the mass of the monomer composition in the mold cavity due to its polymerization. The insufficient amount of supply of the liquid monomer composition from the liquid reservoir into the mold cavity may lead to a high risk of a failure to form the lens blank with desired configuration and size.